Angiogenesis, i.e. the formation of new blood vessels from pre-existing ones, is a complex process which has a fundamental role during embryogenesis, wound healing, and reproductive functions. In a healthy subject, angiogenesis is highly regulated, being turned on for brief periods when required, and then completely inhibited. Failures in the regulation of angiogenesis can have deleterious consequences and many diseases such as atherosclerosis, arthritis, ocular neovascularisation and cancer are typically characterised by persistent angiogenesis.
In the case of solid tumours, the development of an adequate vasculature to deliver nutrients and oxygen to tumour cells is very important. Solid tumour cells depend on angiogenesis for their growth much more than normal cells and tumours can grow progressively only if they can vascularise themselves. Therefore, control of vascular development could permit new therapeutic approaches to cancer.
The Notch pathway is an evolutionarily conserved intercellular signalling pathway which is involved in various biological processes including cell fate determination, cellular differentiation, proliferation, survival and apoptosis. The Notch receptor is believed to play an important part in angiogenesis. There are four different mammalian Notch receptors and several different Notch ligands, including delta-like-1, delta-like 4 (DLL4), Jagged1 and Jagged2.
DLL4 is a transmembrane protein of about 685-amino acids, comprising an extracellular region which contains 8 EGF-like repeats and a DSL domain characteristic of Notch ligands. DLL4 also has a transmembrane domain and a cytoplasmic tail apparently lacking any catalytic motifs. Human DLL4 shares 87% amino acid sequence identity with mouse DLL4. DLL4 is also referred to in the art as “Dll4” and throughout this text “DLL4” and “Dll4” are used interchangeably.
The gene which encodes murine DLL4 has been sequenced and the sequence has been deposited under accession number NM—019454. This sequence is designated herein as SEQ ID NO: 19. The sequence of the corresponding protein can be found under the same accession number and it is designated herein as SEQ ID NO: 20.
The gene which encodes human DLL4 has been sequenced and the sequence has been deposited under accession number NM—019074. This sequence is designated herein as SEQ ID NO: 21. The sequence of the corresponding protein can be found under the same accession number and it is designated herein as SEQ ID NO: 22.
DLL4 can interact inter alia with receptors designated Notch1, Notch2, Notch3 and Notch4. In mice, haploinsufficiency of delta-like 4 ligand results in embryonic lethality due to major defects in arterial and vascular development. DLL4 is therefore likely to play an important part in angiogenesis.
DLL4 activity is primarily regulated at the gene expression level. The main regulator is vascular endothelial growth factor (VEGF) via the phosphatidylinositol 3-kinase/Akt pathway.
The biological function of DLL4 in primary endothelial cells and the expression of DLL4 in renal cancer was investigated by Patel et al. (Cancer Research 2005; 65:19, pages 8690-8697). These workers suggest that selective modulation of DLL4 expression within human tumours may represent a potential anti-angiogenic therapy.
Probably the best-studied and most advanced approach to anti-angiogenic therapy is VEGF inhibition. The humanized monoclonal antibody (mAb) bevacizumab (Avastin) is to our knowledge the only anti-angiogenic antibody approved for the treatment of cancer (Presta L. G. “Humanization of an anti-vascular endothelial growth factor monoclonal antibody for the therapy of solid tumour and other disorder”. Cancer Res., 57: 4593-4599, 1997 and Ferrara N. “VEGF as a therapeutic target in cancer.” Oncology, 69: 11-16, 2005). Bevacizumab was developed from a murine mAb to human VEGF and was selected for clinical development based on preclinical evidence showing high anti-angiogenic and anti-tumour activity (Borgstrom P. “Complete inhibition of angiogenesis and growth of microtumours by anti-vascular endothelial growth factor neutralizing antibody: Novel concepts of angiostatic therapy from intravital videomicroscopy.” Cancer Res., 56: 4032-4039, 1996).
There remains a need for further agents which can modulate angiogenesis, in particular in the context of cancer therapy.